One of the problems encountered in systems employing hydrocarbon fuels is the build-up over time of thermal oxide derived varnish-like deposits on the surfaces of combustion chambers and components of the fuel distribution network. A number of testing methodologies have been used to measure such fuel deposits. For example, the jet fuel thermal oxidation test (JFTOT) is a test which is used to determine the deposit forming characteristics of jet fuels or other fuels. In this test, jet fuel is passed over a heated aluminum tube and deposits accumulate on the tube as a function of fuel quality, time, and tube temperature. The fuel is then given a rating based on the amount of deposit which accumulates on the rod over a fixed period of time. The JFTOT visual rating, using a light box with color standards, is the most commonly used rating method. While this visual rating method is suitable for certain types of fuel deposit evaluations, it has several inherent limitations. The rating scale has a very narrow dynamic range, since a deposit can only be rated into one of six categories (0,1,2,3,4,4+). Furthermore, the results are somewhat subjective, since each operator assigns the rating based on his individual perception of the "best match" to the color standard. For example, a thin, dark colored deposit could be given the same rating as a thicker but lighter colored deposit.
To overcome some of the limitations of the visual rating, a photo optical measuring device, known as a Tube Deposit Rater (TDR) is sometimes used. The TDR eliminates some of the problems associated with operator subjectivity and it allows a much wider rating scale of 0-50 measurement units. However, the TDR method does not overcome any of the other problems common to visual rating, such as the effects of deposit color or texture, and the TDR is incapable of directly producing deposit thickness, volume or mass data.
Another optical method for measuring the thickness of the deposits is based on the use of a transmission electron microscope (TEM). In this method the electron microscope is used to obtain an accurate indication of the actual thickness of the deposit on a cross-section of the aluminum tube. Although the TEM method provides a high degree of accuracy, it is extremely time consuming and expensive. Therefore, it is not feasible as an economical method for routine testing of fuels.
A number of electrical techniques for measuring the thickness of various types of deposits have been shown in the prior art. For example, a prior electrical method for measuring the thickness of an oxide deposit is shown in U.S. Pat. No. 4,495,558, issued to Cath. In this system, the thickness of an electrically insulative metal oxide material deposited on a conductive material is measured by reducing the metal oxide in an electrochemical cell while continuously measuring the cell voltage. A mathematical relationship between the cell voltage change and elapsed time of electrolytic reduction serves as a means for determining the thickness of the oxide coating.
U.S. Pat. No. 3,787,457, issued to Rogers discloses an apparatus and method for measuring the thickness of thin film on a nondestructive basis using an inductive bridge operating in its linear portion of unbalance and with an offsetting potential summed with the output of the bridge and with the bridge being fed with a precise amplitude and frequency signal. This system is calibrated relative to a first known thickness and then succeeding measurements of thin films are related to this known thickness.
The method and apparatus of the present invention, described in greater detail below, provides an effective technique for measuring the thickness of electrically insulative deposits based on the dielectric breakdown voltage of the deposit. In this measurement technique, the dielectric breakdown voltage is used as an indication of the thickness of the deposit. The dielectric breakdown method promises to be an effective, economical means for obtaining a measurement of deposit thickness. One of the difficulties encountered in this method, however, is that the deposit must be electrically insulative. Therefore, the measurements obtained in certain electrically conductive deposits produce spurious data which must be eliminated. The method and apparatus of the present invention provides a means for overcoming this difficulty, as described in greater detail below.